1. Field of the Invention
The present invention relates to a data transmission rate control method and system applicable to cell transfer switches such as ATM (Asynchronous Transfer Mode) switches, ATM routers, ATM bridges, or other telecommunications equipment.
2. Description of the Background Art
Today, the following three methods are predominantly used for controlling the transmission rate of a fixed length of data (referred to as a cell below): the two minimum intercell gap, or cell interval, control schemes or methods, including a cell period based scheme and a clock period based scheme; and the sliding window control scheme. To distinguish the two minimum cell interval control schemes from each other hereinbelow, the cell period based minimum cell interval control scheme is referred to as a cell-based minimum intercell gap control scheme, while the clock period based minimum cell interval control scheme is referred to as a clock-based minimum cell interval control scheme.
First, the cell-based minimum cell interval control scheme will be described. The scheme controls the gap or time interval between the preceding transmitted cell and the current transmitted cell on the basis of the cell transmission period. Accordingly, if a target transmission rate is 50% of the peak rate, for example, the transmission is carried out at two cell intervals.
Second, the clock-based minimum cell interval control scheme is basically the same as the cell-based minimum cell interval control scheme. The clock-based one, however, uses the clock period instead of the cell period to time the interval between two consecutive transmitted cells. This enables a more accurate peak rate control.
On the other hand, the sliding window control scheme makes a decision whether or not a cell can be transferred at each cell timing by shifting a time window with a predetermined length at every cell interval. The transmission of the current cell is allowed if the number of cells in the time window which have already been transmitted is less than a permitted number.
More specifically, reference is made to FIG. 7, which illustrates the sliding window control scheme for controlling the transmission rate at 40% of the peak rate. In the figure, if a cell is transferred at a time period #n-3, the total number of the cells in the time window becomes five which exceeds the permitted number, four. Thus, the transmission of the cell is inhibited at this time period, as depicted with a mark "X". At the next time period #n-2, the transmission of the cell is allowed, as shown with a circle because the total number of the cells in the time window will be within the permitted number, four, even if it is transmitted, because the total number has been reduced by one by sliding the time window by one cell period. Similar processing is iterated at time periods #n-1, #n, and so on.
The foregoing control schemes, however, present the following problems. First, since the cell-based minimum cell interval control scheme controls the transmission of cells based on the cell period, it has a problem in that gaps or intervals between controllable values become rough at high rates (particularly, in a range close to the peak rate). For example, the controllable value is 100% at every cell interval, 50% at two cell intervals, 33% at three cell intervals, 25% at four cell intervals, 20% at five cell intervals, and so on. Thus, the controllable values have greater gaps as the transmission rate increases.
In addition, since the cell-based minimum cell interval control scheme checks each channel in time division mode, its accuracy degrades with the increasing number of channels. For example, assume that the number of channels is 1024, that the check is carried out one channel per clock period, and that one cell period is 53 clock periods; it takes about 20 cell periods to complete all of the cells, which may cause time errors in the transmission control.
Second, although the clock-based minimum cell interval control scheme can solve the controllable gap problem of the cell-based minimum cell interval control scheme, it presents a new problem in that increase in circuit scale is unavoidable as compared with the cell-based minimum cell interval control scheme. This is because the number of bits needed for controlling the transmission enable time increases in the clock-based minimum cell interval control scheme: an increase of six bits is required when one byte is handled during one clock period. For example, when the bandwidth is limited to 1% of that of the peak rate, the transmission must be carried out once per 100 cells. To achieve this, although it is enough for the cell-based minimum cell interval control scheme to have six bits, the clock-based minimum cell interval control scheme requires twice that number, 12 bits, which inevitably doubles the number of bits of its circuit components such as a counter, adder and comparator.
Finally, the sliding window control scheme has a problem in that the width of the shift register increases as the minimum value of the controllable value decreases. For example, although the minimum required window width is only 10 when the minimum value is 10%, it increases, for example, to 100 for 1%, and 10000 for 0.01%. Since the sliding window control scheme requires a shift register with register stages corresponding to the width of the time window, a shift register with an increasing number of bits must be prepared as the minimum value reduces.
The number of bits required per shift register stage depends on a channel identifier and is 24 bits for the VPI/VCI of an ATM cell. Therefore, when the transmission rate of a particular channel is 0.01% of the peak rate, flip-flops of 320 kilobits are required, which presents a problem in that it is difficult to implement such a system in hardware.
Moreover, although it is possible for the cell-based minimum cell interval control scheme or clock-based minimum cell interval control scheme to perform transmission always at a fixed rate independently of the bandwidth because the transmission interval is checked between the previous cell and the current cell, the sliding window control scheme, which determines transmission according to the number of cells in the time window, and hence requires a wider time window as the bandwidth of the channel decreases, has a problem in that the fluctuations of the cells in the time window increase with decreasing bandwidth.